1. Field of the Invention
The present invention relates to an apparatus and a method for measuring a dispersion value of an optical fiber transmission line.
2. Description of the Related Art
With the arrangements of high-speed communications networks, optical communications systems using optical fibers have been built in recent years. In a dense wavelength multiplexing system or a super high-speed optical transmission system exceeding 2.4 Gbps, the wavelength dispersion characteristic of an optical fiber transmission line is an important system parameter. That is, if the wavelength dispersion characteristic of an optical fiber transmission line is inferior, the waveform of an optical transmission signal is deteriorated, which leads to improper reception of the signal on a receiving side. Accordingly, when an optical communications system using optical fibers is configured, it is essential to have good knowledge of the dispersion characteristics of the optical fibers to be used.
Especially, the knowledge of a 0-dispersion wavelength of an optical fiber may sometimes be vital to a system design. The dispersion value and the 0-dispersion wavelength of an optical fiber are standardized by the ITU-T, etc. Although optical fiber makers manufacture optical fibers based on this standard, non-uniformity of manufactured optical fibers cannot be prevented. It is therefore important to measure the dispersion value or the 0-dispersion wavelength of an actual optical fiber.
Conventional techniques disclose that the wavelength dispersion of an optical fiber is obtained by propagating optical pulses through an optical fiber, and measuring the difference between the travel speeds of the optical pulses, which is caused by wavelengths that are different due to wavelength dispersion, that is, the difference between group delay times at a receiving end. The conventional techniques adopt the following two methods.
Phase Shift Method
Preparing for light sources of different wavelengths, and precisely measuring the relative phases of modulated optical waveforms having the different wavelengths after transmission along a fiber.
Interference Method
Measuring an amount of delay time by using spectral white light (low-coherence light) and inserting a fiber to be measured into a port on one side of a Michelson interferometer.
Additionally, a method for making a measurement by using an OTDR (Optical Time-Domain Reflectometer) from both ends of a fiber, is disclosed.
Bidirectional OTDR Method
Measuring a backward scattering light occurring within an optical fiber by using an OTDR, and calculating a measured waveform. The bidirectional OTDR method is referred to in the "Wavelength Dispersion Distribution Measurement Technique for an Optical Transmission Line" by the R&D department of NTT.
With the above described methods, the average of measured wavelength dispersion values over the whole length of a measured fiber, or the distribution in a longitudinal direction is evaluated.
The conventional techniques require the measurement of the difference between wave speeds or the analysis of waveforms, which leads to an increase in a measurement system size (also in a measurement apparatus size).
Therefore, the measurement apparatus is difficult to move, so that the dispersion value of an installed optical fiber is difficult to be measured. That is, even if an optical fiber, after some operation or another, has a dispersion characteristic which is different from the dispersion value measured before installation, it is difficult to again measure the dispersion characteristic. Accordingly, there is a difficulty in suitably adjusting a communications system according to the dispersion characteristic of an installed optical fiber.